1. Field of the Invention
The present invention relates generally to the conversion of precursor fibers to graphitic fibers by pyrolysis, and more particularly to processes for improving surface reactivity of graphitic fibers in order to provide better adhesion of such fibers within a resin matrix so as to obtain superior composite properties, and to fibers provided by such processes.
2. History of the Prior Art
As set forth in detail in a co-pending application of Irvin Wizon, METHODS OF FABRICATING GRAPHITIC FIBERS AND FIBERS PROVIDED THEREBY, Ser. No. 315,658, filed Dec. 15, 1972, now abandoned, and assigned to the same assignee as the present application, carbon fibers can advantageously be converted to a graphitic state with particular fiber characteristics by the use of two or more controlled temperature and concurrent stretching zones. It is shown that the orientation potential of the microfibrillar structure of the fibers has substantial responsiveness to stretching in a lower temperature first zone, which largely controls tensile strength. Some further fibrillar orientation is obtained by stretching in a second thermal treatment zone, but in this zone the control is primarily that of crystallographic order and essentially controls the modulus of elasticity. It is shown that there is an interrelationship between the temperatures and the stretches that are used in the various zones. At the same time, it is observed that the chemical reactivity of the resultant fibers is related to treatment in the two zones, and in particular to the temperatures used in the later treatment zones.
It had previously been thought by workers in the art that a marked reduction in surface reactivity was a necessary incident of precursor characteristics and the graphitic conversion process, over which little if any control could be exercised. Workers in the art have heretofore concentrated on obtaining relatively slight improvements in the surface reactivity of graphite fibers by post treatment of the fibers. A few of the many examples available are found in the use of an acid (e.g. U.S. Pat. No. 3,660,140), oxygen (e.g. U.S. Pat. No. 3,720,536), or a catalyst (e.g. U.S. Pat. No. 3,627,570). Such techniques are specifically utilized to enhance surface adhesion properties. A number of proposals have been made to control the graphitization atmosphere in a selected fashion, for various purposes, as in U.S. Pat. Nos. 3,532,465, 3,723,605, 3,656,903 and 3,116,975. The prior art, however, does not provide a significant basis for substantial improvements in the composite properties of graphitic fibers. The characteristics of a composite using graphitic fibers, for example, are determined not only by the surface adhesion between the matrix and the fibers, but also by other parameters such as the modulus and tensile strength of the fibers. Thus none of the characteristics can be permitted to be significantly degraded, but modulus and tensile must be maintained at desired levels while composite properties such as transverse tensile and shear strengths are markedly improved to meet the needs of the composite structures art.